Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit https://heatinghelp.com/contact-us/.

Zone pump cavitation

dtranidtrani Posts: 25Member
I have a new Alpine install and I'm having trouble with pump cavitation when the supply temp is over ~160 deg. The system seems to be plumbed correctly with a 1" primary, a 1" DHW loop on priority, and two 3/4" baseboard zones (all with Grundfos 3-speed pumps). The pumps on are on the supply side just after the expansion tank. The expansion tank seems to be correctly sized and is working properly (no measurable pressure fluctuation with temp). System pressure is reading ~15 psi.

This is a retro into a system that needs ~170 water to match heat loss on cold days.

The DHW can run at 170 and both the boiler pump and the DHW pump are silent, but during a prolonged heat call both of the zone pumps will get noisy when the set point rises past 160 deg.

Other than raising system pressure, I don't have any ideas. I would re-plumb if I saw an obvious problem but I don't. Any ideas?

Comments

  • GordyGordy Posts: 9,264Member
    System purged? Air separator?
  • dtranidtrani Posts: 25Member
    There's a scoop right at the expansion tank and I am fairly certain the system is purged.
  • hot_rodhot_rod Posts: 11,862Member
    Which pump speed!
    Any restrictive valves upstream of the pumps?
    Sometimes just loosening the flange bolts a bit will relieve the last trapped air bubbles, but a good micro bubble air removal device should handle even the smallest bubbles once the pump gets them to the air separator.

    A pic or drawing?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • GordyGordy Posts: 9,264Member
    As water temps increase air comes out of solution that's why your experiencing this with higher temps. How long has this been up, and running? Raising psi will help keep the air in solution to get to the scoop.
  • dtranidtrani Posts: 25Member
    edited December 2015
    The zone pumps are both set to medium, though they make noise at any setting. This system was installed over the summer and has be doing DHW since. Heat has been for the last 2 months or so. There are no restrictive valves anywhere but the zone loops are squirrelly with lots of 90's to accommodate moving baseboards with renovations over the years.

    The attached pic has the boiler in the background, the primary loop is the middle two pipes heading back, and the supply side is on the right. This was a very cramped install so the plumber had to get creative, but it all looks pretty good to me (except for the unfortunate vertical mounting of the boiler circ, which I will probably have to correct at some point).
  • hot_rodhot_rod Posts: 11,862Member
    I have seen the small discharge port on that air purger plug up, maybe it is not venting air? Sometimes a paper clip will clean out that small port.

    You should not need to, but burping at the flange will grab any errant, rouge air trapped in the pump volute.

    Used to be a screw at the rear end of those pumps to help burp them. In some cases it is hidden under the plastic data plate, just saying :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Paul48Paul48 Posts: 4,492Member
    Are you sure that noise is not coming from the inverted primary circ? Like a dry bearing noise, or growl?
  • dtranidtrani Posts: 25Member
    Pretty sure. Thanks for the suggestions.
  • I hate to ask a stupid question - but "dtrani", can you describe what this suspected cavitation sounds like? Does the sound go away if you throttle the pump to zero flow? Does turning on/off the other pumps have an effect?

    It's pretty hard to cause a change of state (boil) fluid at the eye of the impeller of a small circ in a pressurized system. The cause (if this indeed the case) should (repeat should) be pretty obvious. Restricted inlet would be one.

    I'm thinkin possible a bad sleeve bearing...
  • dtranidtrani Posts: 25Member
    Good question, actually. Sounds like soft knocking and pinging. It's not loud, but you don't have to put your ear right to it to hear it. I'm assuming it's cavitation.

    I haven't tried turning one zone pump off, but I can set either to any of the 3 speeds, including both on low, and while the noise gets quieter at lower speeds, it doesn't go away. Again, only when the water is hotter than ~160 deg.

    Do pumps make noises like this for other reasons, assuming the system is well purged (I will be checking this...)? Is it possible that, since the zones have only recently seen water above 160 deg that I need to give the scoop time to get the last of air out of solution?

    Both pumps are new this season and since they both make noise, but only with hot water, mechanical failure like bad bearings sounds unlikely to me.

  • hot_rodhot_rod Posts: 11,862Member
    Knocking of pinging sounds more like something stuck in the vanes of the impeller. Solder balls, copper reamer shavings are commonly seen in impellers. Maybe?

    I have seen see the small integral checks bounce a bit if you are running right at their threshold. I think something like a .35- .50 psi "pop" on the checks?

    Or shards of teflon tape causing the check to not seat or open completely.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Really can't see "cavitation". At 190 PSI the only way to "boil" water is to put it under a negative gage pressure (partial vacuum). Your noise is on all three speeds which is another reason I doubt cavitation.

    Might be time to take the pump apart and inspect the impeller and rotor assembly.

    The only other way to troubleshoot would be to swap the pump locations and see if the noise stays with the pump or the initial pump location.

    Once the cause of the noise is known I'll bet it will be an "aha" moment... Keep trying and keep posting.
  • hot_rodhot_rod Posts: 11,862Member
    What type of air elimination device? A scoop, or micro bubble eliminator?
    What about the pump in relationship to the expansion tank? Are you pumping away from the tank?

    Here is an example of what goes on in a piping system when you pump away or towards the tank. pumping towards the tank, undertake right circumstances can drop you below NSPH or sub-atmospheric conditions. If it happens at elevated temperatures only, you could be coaxing cavitation.

    Gaseous cavitation is another form, usually prevalent right after a fill before dissolved and microbubblers are purged, not temperature related.

    The min. pressure requirement at the circ increases with temperature, this is the chart Grundfos offers for all the UP series.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Hot Rod - you can't boil water unless it's pressure is below vapor pressure. Period. And that is what cavitation is.
  • hot_rodhot_rod Posts: 11,862Member
    Cavitation is the formation of vapor cavities in a liquid, period. Those vapor cavities can be formed in a number of ways.

    It can and does happen without temperature being involved.

    Wash machine pump cavitate if the wrong soap is used, as do dishwashers with Ivory dish soap :) It's all the foam. Ask the engineers at Askoll.

    Actually your blender can cavitate while making your favorite smoothie, too much protein powder will do it from my experience.
    Vapor pockets form around the impeller, fruit stops moving, rpms increase, it's cavitation, we've all experienced that sound.

    In hydronic circ pumps most often it is caused by temperature, true.

    Gaseous cavitation in hydronics could be defined as: when the liquid pressure at the eye of the impeller drops below the saturation pressure of any of the dissolved gases in the water O2 nitrogen for example, which is why it can occur in a newly filled, cold, hydronic system until the air purger rids the system from microbubbles.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • dtranidtrani Posts: 25Member
    I'm going to drain the system over the weekend to add some baseboard and will take the pumps out and inspect, though I'm not expecting to see much.

    I'm convinced the SpiroVent is working - I purged the zones again and afterward heard it releasing some air.

    I'm starting to think there is air in the system that's coming out of solution at higher temps, since I'm hearing the boiler pump making the same noises (just noticed this), and I can't imaging the boiler pump doesn't have enough suction head...

    My plan is to turn off outdoor reset and let the boiler run at 180 for a while and see if it doesn't work the air out of the system. If that doesn't work I'm going to try raising the static pressure to 20 psi.

    Appreciate the ideas.
  • hot_rodhot_rod Posts: 11,862Member
    Tough to offer more ideas without a drawing or some pictures. With a top quality micro bubble device like you have, complete air removal should only take a few hours to a day, once you fire up the boiler all zones and loops are flowing thru the air eliminator.

    Boosting the fill pressure may help get you over the edge, but really should not be required.

    How high is the tallest point in the system? You want 5 psi pressure at that point. Distance above expansiontank X .434, then add 5 psi to that number.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • dtranidtrani Posts: 25Member
    Total system height is about 30' so I should prob have 18 psi . Pressure is currently set at 15 psi. I don't have any problem driving water through the 3rd floor zone at 15 psi, but I should prob bump it up to 18.
  • hot_rodhot_rod Posts: 11,862Member
    Any float type air vents, or any air vents at the high points? On a radiator or at a baseboard ell?

    With a hot system, pump off, bubbles will migrate to high points. If you have a lot of ups and downs in the piping, air removal requires a bit more work and planning.

    You need to have at least 2 fps velocity in the piping to shove air back to the Spiro. this is especially important with tall vertical piping. Low flow velocity can allow bubbles to rise up even with downward flow.

    This chart shows you what flow rates you need to establish the velocity needed for pushing air, and heat around.

    Bumping up the pressure, and speeding up the circ temporarily may help, if in fact you have an air problem causing the noise.

    You should adjust the air pre-charge in the expansion tank to match the 18-20 psi fill, if you go that route.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • dtranidtrani Posts: 25Member
    Problem solved.

    It seems like the cold static pressure of the system was actually about 12 psi. I adjusted up to 18, and now I can run at 180 deg with silent pumps.
  • hot_rodhot_rod Posts: 11,862Member
    "Pressure, pushing down on you, pushing down on me."

    Just keep that 1980's song Under Pressure, by Queen and David Bowie in mind when troubleshooting pressurized hydronic systems.

    Keep an eye on the pressure when the boiler is at high limit temperature. By bumping up the fill and expansion pre-charge pressure, you have reduce the acceptance volume of the expansion tank. You may need to upsize, or add an additional tank if it was sized closely.

    Hopefully you have an accurate pressure gauge in the system.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • Mark EathertonMark Eatherton Posts: 5,843Member

    Hot Rod - you can't boil water unless it's pressure is below vapor pressure. Period. And that is what cavitation is.

    Tell my 75 HP boat motor that. The pitch was intended for a lighter boat and speed. On my pontoon boat, I have to be REAL careful to not allow it to cavitate. If I punch it out of the hole, I am guaranteed it will cavitate. If I inch it up slowly, no problem. And the lake is C O L D.
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
Sign In or Register to comment.

Welcome

It looks like you're new here. If you want to get involved, click one of these buttons!